1. Field of the Invention
The present invention applies generally to variable reluctance transducers employing parallel resonant circuits producing signals for manipulation to represent the input to the transducer with an output value. Specifically, the present invention employs digital control of parallel resonant circuits incorporating variable reluctance elements acting as the force sensor in the transducer providing dual frequency signals for digital calculation to obtain a quotient of the frequencies thereby substantially eliminating the resonant frequencies as a variable in accuracy of the device. Manipulation of the produced quotient through digital calibration tables responsive to a temperature sensor produces an output value which substantially eliminates temperature-induced errors in the system and further increases accuracy.
2. Prior Art
Variable reluctance transducers have been in use for a number of years in the measurement of physical parameters which can be resolved into force on a diaphragm. Displacement or deformation of the diaphragm due to the force which may be fluid pressure, acceleration or other physical phenomenon exerted on the diaphragm provides a basis for measurement. An inductor or capacitor mounted to the diaphragm changes value based on the displacement or deformation. Incorporation of the inductor or capacitor in a resonant circuit allows measurement of the deformation by change in frequency. The use of two variable reluctance elements to produce two separate frequencies based on deformation of the diaphragm provides a basis for calibrating and correcting the measured value. Integration of both variable reluctance elements with the diaphragm to produce opposite variation of the parameter value or fixed mounting of one element for comparison with the element mounted to the diaphragm allows calibration through various combinations of the two frequencies produced. U.S. Pat. No. 3,140,450 entitled "Force Controlled Variable Reactance Transducer," issued on Jul. 7, 1964 to the inventor of the present application discloses the structure of such a transducer wherein the frequencies produced by the two resonant circuits are mixed to obtain a difference frequency for measurement of the force applied to the diaphragm.
Calibration and compensation of variable reluctance transducers has been accomplished in the prior art through the design of the oscillator circuit provided to drive the variable reluctance resonant circuit of the transducer. Various means to provide constant amplitude signals for increased accuracy of measurement are disclosed in various prior art patents such as U.S. Pat. Nos. 3,763,444, 3,837,227 and 4,947,139, all issued to the inventor of the present application.
While significant accuracy improvements have been made in variable reluctance transducers, the inaccuracies caused by nonlinearity in deformation characteristics of the diaphragm, influence of temperature on the diaphragm characteristics and electronic components, and conversion of the data produced by the transducer to usable form continue to require creative solutions.
The present invention provides a novel oscillator circuit for control of switchable dual resonant circuits for a variable reluctance transducer in combination with a digital control system for accurate conversion of the raw transducer signal and to provide a digital output of desired form allowing easy analysis and manipulation of the data produced.